Best of AOP - Planetary Nebulae

NGC 6781

This is an almost a perfect bubble of gas cast off by a single star that has died. The bubble continues to expand; it is approximately two light years across now. Its imperfection (from our perspective) is due to photodissociation (yes... this is a real word) by interstellar UV radiation. Energetic photons from nearby bright stars decompose the gas bubble. Note the dim central blue star (a blue/white white dwarf).